The present invention relates generally to laser-based systems useful in construction applications and, more particularly, to a laser-based system and method for measuring the X-Y coordinate of a point in a continuously changing frame of reference.
In the construction industry, the level of automation and robotization is very low. One reason for this is the difficulty of properly positioning machines and tools. In the construction of commercial buildings, for example, various points of reference have to be established, such as placement of floors, ceilings, electrical wiring, plumbing, and heating and cooling ducts. Establishing reference points is time consuming and expensive, particularly as such work is often contracted out to companies which specialize in this work. Moreover, for many applications, the machine has to move toward the product. When the reference point is continuously changing, the difficulty of positioning the machine is compounded.
In prior laser-based systems, such as that disclosed in U.S. Pat. No. 3,588,249, for example, a reference plane is established throughout a work site by a transmitter which emits laser energy in the level reference plane. The laser beam is projected radially outward from the transmitter and rotated continuously through 360 degrees to sweep around the entire work site. One or more receivers may be employed throughout the work site to sense the location of this reference plane. Such receivers may be mounted on a surveyor's rod, as described in U.S. Pat. No. 4,030,832, or they may be employed as part of a control system for construction or agricultural equipment, as described in U.S. Pat. Nos. 3,813,171; 3,873,226; 3,997,071; and 4,034,490.
In order to track the movement of a reference point, prior art laser systems, such as the laser survey system disclosed in U.S. Pat. No. 4,830,489, have provided not only elevation information, but also position information in two other axes. The system includes a laser transmitter, located at a reference position at a work site, which sweeps a laser beam radially in a reference plane. The system includes a receiver, located on mobile earthmoving equipment operating at the work site. The receiver has a sensor that determines the relative elevation of the laser reference plane. The receiver also includes a pair of reflectors, each of which reflects laser energy back to the transmitter. The laser transmitter also has a sensor which receives the reflected laser energy, and, in response thereto, produces receiver position information for transmission to the receiver.
The laser transmitter is designed to rotate the laser beam continuously through 360 degrees at a substantially constant angular velocity and thus sweep the beam past the two reflectors of the receiver once during each revolution. During each revolution of the laser beam, the transmitter receives back two short bursts or pulses of laser energy from the two reflectors. Thus, since the laser beam sweeps at a substantially constant angular velocity and the distance between the reflectors is fixed, the time period between receipt of these two pulses provides an accurate basis for the calculation of the range or distance of the receiver from the transmitter. However, since the accuracy of the range calculation is dependent upon a uniform rotational velocity for the laser beam, any variability in the rotational velocity will decrease the accuracy of the range calculation.
Three-dimensional laser based systems are generally complex and expensive. It is sometimes necessary and, often, sufficient to know only the exact location of a reference point within a two-dimensional framework. A method for locating a reference axis of a mobile robot vehicle in a two-dimensional structured environment is disclosed in U.S. Pat. No. 4,796,198. In that reference, a laser carried by the mobile vehicle generates a rotating beam which strikes retroreflectors located at the periphery of the structured environment. Computation means compute the location of the mobile vehicle based on the location of the retroreflectors and the reflected light. In a 50 foot by 50 foot structured environment, for example, the location coordinates of a moving vehicle can be determined to an accuracy of approximately 1 inch. However, construction industry accuracy requirements often call for accuracies within a centimeter or a few millimeters. In addition, many applications require not only the X-Y position of a point, but also an indication of the direction in which the point is moving or is to be moved.
It is seen then that there is a need for a positioning system and method wherein highly accurate position coordinates and orientation of a point, which are insensitive to deviations in the rotational velocity of the rotating light beam, can be determined.